Electronic circuit protector



G. A. ROBERTS.v

f original Filed July 23 1962 v ELECTRONITC CIRCUI'T PROTECTOR 4Sheets-Sheet l INVEITR.

@Erjon y/S mZerzZs 1 7, .1967 G. A. ROBERTS ELECTRONI C C IRCUITPROTECTOR 4 Sheets-Sheet 2 original Filed July 2:5, 1962 .//5 me; a c P56er/ser r l I 63a??? 277774Z )rfa/waff??? 147,1 1967 x G, .A. ROBERTS3,299,322 l l ELECTRONIC CIRCUIT PROTECTOR f original Filed July 25'1962A 4 sheets-sheet ffm@ 1427274 e, l 62727006715@ e] .y

I I I 'I mmf-mf I I I I I Jan. 17,4 1967 G. A. ROBERTS 3,299,322

,. l ELECTRONIC CIRCUIT PROTECTOR voriginal Filed July 23; 1962 4sheets-sheet Wim/J 'i @0176/4 INVENTOR.

/vEKsf nted States latent 3,299,322 ELECTRONIC CIRCUIT PROTECTGR GordonA. Roberts, Ann Arbor, Mich., assignor to Mechanical Products, Inc.,Jackson, Mich., a corporation of Delaware Continuation of applicationSer. No. 211,668, July 23, 1962. This application Aug. 9, 1965, Ser. No.482,990 5 Claims. (Cl. 317-33) This invention relates generally toelectrical apparatus, and more particularly to an electronic circuitprotector. This application is a continuation of my co-pendingapplication Serial No. 211,668 filed July 23, 1962, for ElectronicCircuit Protector, now abandoned.

Conventional circuit breakers generally fall into two categories,namely, those having a relatively long time delay between overload andtrip, and those having a relatively short time delay. The time requiredfor both types of circuit breakers to trip upon the occurrence of arelatively small overload is longer than the time required to trip dueto a large overload. Thus, the difference between the two types lies inthe relative time scales for these inversetime characteristics, theinstantaneous trip circuit breaker having a greatly compressed scale.

The current responsive elements employed in conventional circuitbreakers are also of two general types, namely, thermal and magnetic.Thermal elements inherently have a relatively long time delay and aretherefore, generally used in circuit breakers which exhibit a pronouncedinverse-time characteristic. The time delay of a magnetic element isrelatively shorter than that of the thermal element, resulting in theuse of such magnetic elements in the so-called instantaneous tripcircuit breakers. However, a longer time delay is often built intomagnetic circuit breakers in order to achieve a more pronouncedinverse-time characteristic.

Certain types of loads, for example, wiring and devices having thecharacteristics of a wire, exhibit an overload characteristic which isessentially inversely related to time following a curve comparable tothat of conventional time delay circuit breakers. Therefore,conventional protectors can substantially match the overloadcharacteristics of loads of this nature and thereby provide a reasonablyhigh degree of protection for such loads.

However, the problem of protecting the electronic loads, or, morespecifically, electronic loads having a transformer input, has created aneed for a different approach to circuit protection. Conventionalcircuit breakers fail to adequately protect such electronic loadsbecause of the presence of large turn-on current transients in suchloads and because of the need for close-n protection and rapid trippingduring normal operations.

In an electronic circuit using a transformer input power supply, a largecurrent transient may flow through the transformer primary when theswitch is closed, due to the degree of saturation of the transformercore. The occurrence of such a transient is therefore random variable.The only way, heretofore known and used, to protect electronic equipmentof this type without excessive tripping of the associated circuitbreakers has been to employ an inverse-time delay circuit breaker whichis rated at a high enough current rating to allow the transient topass.The inverse-time characteristic is necessary in order to tolerate theshort but relatively large turn-on transient. Due to this samecharacteristic, however, the circuit breaker will exhibit a relativelylarge time delay before tripping upon the occurrence of an overload ofsmall magnitude. Therefore, such a breaker is not capable of protectingthe circuit against small overloads of relatively short duration,sensitivity and speed of operation having been sacrificed in order totolerate the turn-on transient.

If a short time delay is used, a higher rating will b e required to holdthe transient, with the result that the ultimate trip point may beseveral hundred percent higher than the normal load current. Here,sensitivity is sacrificed for speed of action. Thus, for a given loadcurrent and transient, a satisfactory compromise may be impossible.

The foregoing problem is not as series if the circuitry utilizes vacuumtubes, in that such tubes inherently have relatively large overloadtolerances. In contrast, semiconductors may be destroyed by onlymomentary overloads, if these overloads exceed the ratings of thesemiconductor components. Unless highly overrated components are to beused, an improved system of circuit protection must be incorporated.

The broad object of the instant invention is a solution to the problemsincident to protecting the class of loads characterized as transformerinput electronic circuitry. The electronic circuit protector of theinstant invention solves these problems by offering protection in areaswhere conventional circuit breakers fail because of their basiccharacteristics. The electronic circuit protector accommodates anytransients present at turn-on, thereafter affording fast acting andclose-in protection during normal operation.

The operating characteristics of a conventional circuit breaker are, ingeneral, dependent on load current, and are therefore not correlatedwith the turn-on of the load. The time required to trip a breaker thathas been carrying full load current is slightly less than the timerequired.

for the same overload current applied to an unloaded breaker.

The electronic circuit protector of the instant invention presents atrip characteristic that is correlated directly to turn-on time, andwhich is independent of the magnitude of load current. The electroniccircuit protector changes its characteristics with respect t0 time, timebeing measured from turn-on of the load.

In other Words, the relative size of the current required to trip theelectronic circuit protector changes with time, but the time requiredfor the electronic circuit protector to trip on said overload isindependent of the margin by which the overload exceeds minimum tripcurrent.

As an example of the foregoing discussion, consider an electronic powersupply employing an input power transformer and having semi-conductordiode rectifiers in the secondary thereof. It is a characteristic oftransformers that large turn-on current transients may occur when poweris applied to the transformer. Such transients may be 10 to 20 times thenormal load peak current. It is desirable for the protective device forsuch circuits to pass the large current transients without opening thecircuit, yet give close-in fasttripping protection upon the occurrenceof subsequent overloads of relatively small magnitude. The turn-ontransient may be 1000 to 2000% of normal load, yet the circuit protectorshould trip out upon the occurrence of overloads of, for example, twopercent within a cycle or less during normal operation.

If a conventional so-called instantaneous type circuit breaker is usedto protect such a circuit, its rating has to be close to 1000 to 2000%of normal load to tolerate the possible turn-on transients. Obviously,such a conventional circuit breaker does not provide adequate protectionto load components during subsequent normal operation unless they alsoare designed to tolerate 1000 to 2000% overloads.

If a conventional inverse time delay circuit breaker having a relativelylong time constant with, for example, -an ultimate trip level of ofrated load is employed so as to tolerate the turn-on transient, the timeconstant or degree of time delay required in such a circuit breaker willbe a function 'of its characteristics and the turn-on transient energy.During normal operation, such a cir- 1 cuit breaker will eventually tripon overloads of, for example, 30% or more. However, because the timerequired to open the circuit upon the occurrence of such an 'overloadmay be quite long, for example, a minute or more, semi-conductorcircuits may be damaged. Thus, in conventional circuit breakers, thereis an inverse relationship between the ability to tolerate turn-ontransients and the ability to give subsequent Iprotection.

Accordingly, another object of the instant invention is an electroniccircuit protector that is able to tolerate large turn-on transients, yetsubsequent to the occurrence of the turn-on transient, offer close-infast trip protection.

Another object of the instant invention is an electronic circuitprotector that provides an independent selection of protectioncharacteristics, depending upon the requirements of the load at anyparticular time.

Another object of the instant invention is an electronic circuitprotector wherein a trip point can be set relatively close to normalrated load of the device being protected.

Another object is an electronic circuit protector that has a relativelyshort t-rip time on lower overloads.

Another object is an electronic circuit protector that tolerates largeturn-on transients.

Another object is an electronic circuit protector having a stable trippoint.

Another object is an electronic circuit protector having a continuouscalibration adjustment.

Another object is an electronic circuit protector that is remotelycontrollable.

Another object is an electronic circuit protector having a programmabletrip set that is time or load dependent.

Another object is an electronic circuit protector that is insensitive toshock and vibration.

Other objects and advantages of the instant invention will be apparentin the following specification, claims and drawings wherein:

FIGURE 1 is a block diagram of an electronic circuit protector of theinstant invention;

FIG. 2 is a circuit diagram of a power supply suitable for use with theelectronic circuit protector;

FIG. 3 is a left-hand portion of a circuit diagram of the electroniccircuit protector;

FIG. 4 is an intermediate portion of the circuit diagram of theelectronic circuit protector; and

FIG. 5 is a right-hand portion of the circuit diagram of the electroniccircuit protector.

The electronic circuit protector of the instant invention consists 'oftwo distinct parts; namely, a switch and a governing control circuit forthe switch.

The function of the control circuitry is two-fold; (1) the controlprovides a manual on-off operation of the switch, and (2) it providesfor overload trip opening of the switch caused by an overload in anelectric circuit containing the electronic circuit Iprotector. The l-oadconditions that constitute an overlo-ad are defined by the control andmay be time varying.

One of the important features of the electronic circuit protector iscentered around the control function. This control overload tripoperation can be analyzed in two steps: (a) sensing the overload, and(b) producing the trip.

Referring now to FIGURE 1 of the drawings, there is shown, in blockdiagram, an electronic circuit protector in accordance with an exemplaryconstructed embodiment of the instant invention. The electronic circuitprotector comprises a control circuit and a controlled switch.

The controlled switch is electrically connected to the control circuitand to the load. The load is connected to a source of electrical energyas well as to the control circuit.

The control circuit comprises a senser, for example, a shunt resistancehaving an output indicative of conditions in the electrical circuitcontaining the electronic circuit protector and load. The output of thesenser is amplified by a Variable gain amplifier which then drives ageneralized function circuit. The output of the generalized functioncircuit is fed into a threshold circuit. If the generalized functioncircuit produces an output exceeding a certain specified thresholdlevel, a trip signal is fed to a switch control circuit. The switchcontrol circuit, in addition to controlling the controlled switch, feedsinformation to a generalized gain control function which determines howthe variable gain amplifier is to vary with respect to time or othervariables that may be present. The switch control circuit also usesinput information from an input, on-ofi-reset switch to effect controlyof the input from the threshold circuit.

If, for example, the input `on-off-reset control switch is turned to theon state, and in turn, turns on the controlled switch, then load currentflows through the current senser, and the controlled switch to the load.

In addition to turning on the controlled switch, the switch controlcircuit also effects control of the generalized gain control functioncircuit and through this circuit, the variable gain amplifier.

The manner in which the variable gain amplifier is to vary with respectto time and other input control variables will be dependent upon theparticular load and the resultant characteristics of the electroniccircuit protector. Control is obtained from the characteristics of thegeneralized gain control function, which would generally be tailored tothe particular load. For example, the variable gain amplifier mightstart at zero gain at zero time, where zero time corresponds to turn-ontime, and then gradually change gain until full sensitivity was achievedat some later time such as milliseconds, after turn. From this time on,the gain would remain constant until a new on cycle was initiated.

The generalized function circuit performs some functional operation onthe variable gain amplifier output. For example, the function may befull wave rectification. The function might be the determination of theR.M.S. value of the wave form for each preceding half cycle at the endof each half cycle, with no averaging or carryover from half cycle tohalf cycle. Beyond these functions, there are many other possibilities.The particular function or functions selected would depend upon the loadand the protection criteria for the load.

The threshold circuit senses whether the output of the generalizedfunction circuit exceeds or does not exceed the critical level set. Thethreshold circuit may be fixed, or the threshold circuit may be afunction of some variable, such as line voltage.

It is to be noted that the generalized gain control function could takeinformation from the load and modify the generalized function so thatthe trip point is varied as a function of load conditions. For example,in a power system, there may be a requirement for sustaining operation,event though an overload exists. Yet, under other conditions, it may bedesirable to trip out, due to an overload of the same magnitude.Variables of this nature can be controlled with the generalized gaincontrol function.-

One of the other desirable functions that might be performed in thegeneralized function circuit is the production of an inverse timecharacteristic where such a characteristic is desired. Such an inversetime characteristic is not to be confused, however, with the inversetime characteristic of conventional circuit breakers heretofore knownand used necessary to allow initial turn-on transients to pass.

The turn-on cycle of an exemplary disclosed embodiment of the instantinvention may be broken down into three phases or states for the purposeof explanation. It is to be understood that other modes of operation arecontemplated, the use of three relatively distinct phases being dictatedby a particular load to which the exemplary constructed embodiment wasapplied.

In state one, trip will not occur regardless of the magnitude of anoverload. This feature is obtained by completely inhibiting thethreshold circuit during this period.

During state two, the gain of the variable gain amplifier is programmedrelative to the turn-on time, resulting in a period of desensitization.

State three is the normal operating mode of the electronic circuitprotector at which time it operates at maximum sensitivity.

During state two and three, an overload occurs, which, when combinedwith the load wave form, exceeds the normal full load peak, a tripsignal is generated, producing grip at the next current zero. If theoverload occurred latein the first half cycle so that the combined valuedoes not exceed peak value, then the switch will not trip until the nextsubsequent current zero. For a given overload and state ofturn-on-cycle, trip is also infiuenced by a factor which may be termedthe trip mode.

Line voltage variations result in load current variation, which, in theyabsence of compensation, may be sufficient to trip the switch, eventhough no overload has occurred. If this is undesirable, the electroniccircuit protector can be line voltage compensated, thus eliminatingspurious trip. The electronic circuit protector thus has two modes ofoperation, (l) a constant current mode which trips whenever the currentexceeds a preset constant level, and (2) a compensated mode which tripswhen the current exceeds a level which is proportional to line voltage.It should be noted that compensation with other than linearcharacteristics can be incorporated, for example, the electronic circuitprotector can be compensated for ambient temperature.

SENSER As best seen in FIG. 3, the current senser, in an exemplaryconstructed embodiment of the electronic circuit protector, comprises aplurality of shunt resistors R1, R2, R3 and R4 in series with the load.It is contemplated that other types of sensers having an outputindicative of circuit conditions may be employed, the shunt resistorsR1, R2, R3 and R4 being exemplary in nature.

VARIABLE GAIN AMPLIFIER The output of the current senser is fed to thevariable gain amplifier which comprises a photocell PC having a variableresistor VRl and a fixed resistor RS in series connection therewith `anda variable resistance VRZ and fixed resistance R6 in parallel, all ofwhich constitute input resistive elements to an operation amplifier A1.The amplifier A1 has variable resistor VR3, a fixed resistor R8, and acapacitor C1 connected thereacross.

Basically, there are two input resistor networks to the operationalamplifier All, one of which consists of the variable and fixed resistorsVR2 and R6, and the other of which consists of the photocell PC inseries with the variable and fixed resistors VRI and R5.

The capacitor C1 and variable resistor VR3 and fixed resistor R3function as a feed-back around the amplifier A1. VR3 and R8 are the lowfrequency gain filter elements. The function of the capacitor C1 is toprovide a high-frequency cutoff point so that noise and high frequencyinterference that might get into the input is reduced Iat the output.

The relationship between the input and feed-back resistance networks issuch that the gain equals the ratio of feed-back resistance to the inputresistance, so that either the input resistance or the feed-backresistance can be changed to change the gain. For the purposes ofcalibrating the equipment, the variable resistor VRS in the feed-backnetwork is used as a gain control. When the feed-back resistance isfixed, further gain control can be obtained by changing the resistanceof the input resistor network.

As will be discussed in greater detail, when the electronic circuitprotector is in state two, which is the desensitized state, thephotocell PC is dark, and the resistance thereof is relatively high.Thus, since the resistance of the photocell PC is relatively largecompared with the parallel path of the resistors VRZ and R6 during thedesensitized state, gain is determined by the particular setting of thevariable resistor VRZ, plus R6 relative to the resistance of thefeed-back network.

When the circuit changes from state two to state three, a lamp L-1 inthe generalized gain control function circuit is energized. The lamp L-1functions as an on or off control by effecting a change in theresistance of the photocell PC from a high value to a relatively lowvalue. It is to be noted, however, should it be desired to vary gain asa function of time, that the intensity of the lamp L can be controlledso as to produce a specified resistance in the photocell PC in order toget a specified gain.

When the lamp L attains full brightness, the resistance of the photocellPC is minimized. When this condition obtains, resistance of thephotocell PC is relatively small in comparison with the resistance R6plus VR2 and PC plus VRI plus R5 in their relationship to the feed-backnetwork resistance primarily determines gain.

The variable resistor VRI functions as a trip set control which is usedto set the actual trip level during state three or the normal state ofthe electronic circuit protector 6. It may be manually controllable soas to provide a trip set range from minus 10% of rated current to plusoverload. The variable resistor VR-Z is the control that sets the trippoint during the desensitized interval or state two. The variableresistor VRS in the feed-back network is a fine calibration control.

GENERALIZED FUNCTION CIRCUIT The generalized function circuit effectsfull wave rectification of output from the variable gain amplifier inthe exemplary example. It is to be noted, however, that the generalizedfunction circuit can be used to generate all sorts of other functions,e.g., anaverage value of the full wave rectified input current, orwhatever the input variable might be. Alternatively, it can be used togenerate an average value over half-cycle intervals or over severalcycle intervals. Alternatively, the generalized function circuit canproduce the R.M.S. value of the input wave form over, for example, aninterval of a half-cycle or longer.

As `best seen in FIG. 3, the generalized function circuit comprises aninput resistor R10 that feeds an amplifier A2. A feed-back resistor R12is connected across the amplifier A2. In order to reduce noise and highfrequency response beyond a certain point, a capacitor C2 is connectedacross the feed-"back network. A pair of diodes D1 and D2 constitute therectifiers, one of which is fed with an in-phase signal and one of whichis fed with an out-of-phase signal. The outputs of the diodes D1 and D2are connected to a load resistor R14.

THRESHOLD CIRCUIT The output of the generalized function circuit is fedto the threshold circuit. The threshold circuit compares theinstantaneous value of the output from the generalized function circuitwith a specified threshold level. The threshold level may be a variablethat is dependent upon a function which is fed into the thresholdcircuit, or it may be fixed. Comparison of the input voltage to areference voltage is effected by a cascade difference amplifiercomprising transistors T1, T2, T3 and T4. A reference linput voltagewhich, in the present system, is a D.C. voltage, comes in through areference selector switch S1. The D.C. voltage may be a constant orrelated to, for example, input line frequency. If the input to the baseof the transformer T2 is of a certain specified level, and if the inputto the base of the transistor T1 is of relatively srnaller voltage, thethreshold circuit does not produce a trip signal. The threshold circuitbasically is a difference amplifier, the output of which is a nonlinearfunction that is produced -by a very small difference between the inputand reference voltages.

As the input level from the generalized function circuits is increasedto a point where it balances the reference sign-al, the thresholdcircuit produces a response that 1s an output trip signal. This outputtrip signal appears across a resistor R24 to the -base of a transistorT7 in the initial inhibiting circuit.

INITIAL INHIBITING CIRCUIT The initial inhibiting circuit is seen inFIG. 4. The initial inhibiting circuit completely inhibits, for aspecified period of time, the turn-on cycle. This is arfunction that ispart of the generalized gain control circuit but 1s more easilyinfluenced at this point.

As best seen in FIG. 4, the initial inhibiting c1rcu1t comprises atransistor T5 which is fed through a pair of resistors R40 and R42 so asto function as a switch.

The transistor T5 has in its collector circuit a capacitor C-S which iscoupled to the base of a transistor T6. The transistors T6 and T7constitute a series and circuit which is the logic circuit used forinhibiting. Thus, even though a signal is applied to the transistor T7from the resistor R24 in the threshold circuit, there is no outputsignal therefrom if the lower transistor T6 is cut oft". The function ofthe transistor T5 and the coupling capacitor C8 is to put a signal intothe base of the transistor T6 such as to cut it off for roughly thefirst two cycles. This constitutes a timing network based upon therelationship of the capacitor C8 and the resistance in the circuit,thereby to determine how long the transistor T6 will be cut off. Avariation in the number of cycles that the transistor T5 is inhibitedcan be effected by changing the value of the capacitor C8, or thecircuit resistance.

The transistor T5 is controlled lby the input on-off-reset control whichcontrols turn-on. Thus, the initial inhibiting circuit only operates forthe first two cycles after the electronic circuit protector has beenturned into its on state.

INPUT ON, OFF, RESET CONTROL As seen in FIG. 4, the input on-off-resetcontrol comprises a single throw, double throw switch S2 that effectsmanual control of on, off, and reset of the entire system.

SWITCH CONTROL CIRCUIT As seen in FIG. 4, the switch control circuitcomprises a pair of flip-flop networks designated as on-off and trip Theon-off flip-flop network controls the state of the controlled switch 10.The trip flip-flop network controls a trip indicator lamp L-4 whichindicates when the electronic circuit protector is in the tripped state.

The on-off flip-flop is fed from an off terminal of the inputon-off-reset switch S1 through a diode D4 to the collector of atransistor T10 that forms the off side of the on-oif flip-flop,Similarly, the off terminal of the switch S2 is connected through adiode D8 to the collector of a transistor T-11 forming one'side of thetrip flip-flop. Thus, when the input on-off reset switch S1 is in theoff state, the on-off ip-op is forced to the off condition and the resetflip-flop is forced to the reset condition. When the on-off reset switchS1 is switched to the on state, a pulse is generated. The pulse, througha diode D3, flips the on-off flip-flop 132 into the on state. This pulseis generated by a resist- Aance capacitance network comprising a pair ofresistors R50 and R52 and a capacitor C9.

As the capacitor C9 builds up a charge through the resistor R52, theon-off flip-iiop is decoupled from the switch S2 by the diode D3. Eventhough the switch S2 is in the on state, an internal off signal fromsome source other than the switch S2` can turn the on-off iiipflop tothe off state. Thus, the electronic circuit protector of the exemplaryexample is trip-free in operation.

A plurality of diodes D4, D5, and D6 couple into the ,on-off Hip-flopwhich are not coupled to the on-oif switch S2. The function of the diodeD6 is to couple a trip signal from the threshold circuit and passed bythe initial inhibiting circuit to the on-off flip-flop.

It is to be noted that a trip signal to the diode D6 is also fed to adiode D9 in the trip flip-iiop. Thus, when a trip signal occurs, theon-off flip-flop 132 and the trip yflip-flop move to the off and trippedstates, respectively. It should be noted that this is different fromwhat occurs when the `on-off reset switch is turned off, at which timethe on-ofr" flip-flop 132 shifts to the off state, but the tripflip-flop 134 is shifted to the reset state. Thus, depending upon thetype of off signal to the on-off Hipflop, the trip Hip-flop goes to thetripped or freset condition.

T he diode D5 couples into the on-off flip-flop from the trip indicatorflip-flop and performs what may be termed a redundant function. When,for example, power is applied tothe system when the switch S2 is in theon state, the trip indicator may go to the tripped state. If thisoccurs, then the on-off flip-flop should be off. Thus, the t-ripindicator flip-flop is coupled to the ori-off flip-Hop through the diodeD5 to insure that the on-off flip-flop goes to the off state when thetrip indicator Hip-flop is in the tripped condition.

The diode D7 is coupled to the off side of the on-off flip-flop from thegeneralized gain control function. If the lamp L-l in the generalizedcontrol function should burn out, the life thereof being relativelyshorter than the life of the other components of the electronic circuitprotector, the variable gain amplifier is left in the desensitizedstate. It is desirable for the electronic circuit protector to be shutoff if there is such a lamp bulb failure. Thus, the function of a signalcoming into the diode D7 is to shift the on-off flip-flop to the offcondition.

The on-off flip-flop has a pair of transistors T14 and T15 which areused to switch a pair of off and on lamps L-Z and L-3, respectively.When the on-off ipflop is in the off state, the transistor T10 isconductive, thereby turning off the transistor T5 in the initialinhibiting circuit, and vice versa. If T5 is off (which corresponds tothe electronic circuit protector being off) and if R46, R48, C8, T6 arein an electrical steady state, then the initial inhibiting circuit couldpass trip signals. However, if such signals were to pass, they would notinfluence the on-oif Hip-flop since it is already off. Assuming that T5is turned on, C8 will instantaneously hold its charge or voltage, thusT6 is turned off, which inhibits any trip signals. As soon as T5 isturned on, C3 begins to charge, the rate of charge is determined by theequivalent resistance of the discharge path. This is primarily R48. Atsome predictable time after T5 is turned on, the charge on C8 will havecharged sufficiently to again turn on T6. The timing for this operationin the exemplary mode is about 32 milliseconds. After T6 turns on, tripsignals are no longer inhibited. Thus, this circuitry inhibits tripsignals for about the first two cycles `after turn-on at 60 c.p.s. linefrequency.

GENERALIZED GAIN CONTROL FUNCTION It is desirable to tu-rn on the pilotlamp L-l in the generalized gain control function thereby to turn on thephotocell PC at a prescribed time in the sequence of events of thesystem. After the completely inhibited interval of, for example, twocycles, there is a desensitized interval of approximately 60 to 180milliseconds, after which period the electronic circuit protector.changes from the desensitized state to a sensitive state.

It is to be noted that this time sequence runs from turn-on time of theelectronic circuit protector. Since L1 controls the shift from thedesensitized state to the sensitive state, it is desired to turn thelamp L-l on at some predetermined time subsequent to when the system isfirst turned on. If an extremely short time delay is all that isrequired, the time delay characteristic of the lamp L-l, namely, itsthermal lag incident to heating up, may be adequate. However, if asomewhat longer delay is desired, a delay circuit must be provided.Thus, the generalized gain control circuit performs three functions;

(1) to perform a delay which constitutes a desensitized period beforeturn-on of the lamp L-l, (2) to turn the lamp L1 on yand insure that itwill remain on subsequently as long as the system remains in the onstate, and (3) to sense the condition of the lamp L-l when it is on. Ifthe lamp should be on but has failed, then to turn off the on-oifflip-flop.

As seen in FIG. 3, the lamp L-1 is connected between the emitter of atransistor T-22 and a resistor R86. The other side of the lamp L1 isconnected to the collector of a transistor T23. The transistor T23 is aswitching transistor that determines whether current flows to the lampL-1 or not. The resistor R86 is a current sensing resistor that senseswhether or not there is lamp current owing.

The collector of the on transistor T12 in the on-off flip-flop iscoupled into the generalized gain control function through a capacitorC23 and diode D10. The capacitor C23, in conjunction with a resistorF102, constitute a differentiating network that couples to a single shotmulti-vibrator circuit. The single shot multi-vibrator provides the 60to 180 millisecond time delay or desensitized period referred tohereinbefore.

The diode D12 is coupled to a Variable resistor VR-S which providesinternal adjustment of the time delay.

The output of the single shot multi-vibrator is connected to the diodeD11. An output of the on-of liipop is connected to the diode D10. DiodesD and D11 constitute an and circuit. In its stable state, fthe singleshot multi-vibrator has T25 conducting, thus D11 is back biased. At thesame time, if the on-oi flip-flop is off, then D10 is conducting, T24,T23 and L1 are off. If the on-off flip-flop is turned on, then theunstable `state of the single shot multi-vibrator is initiated and T25becomes non-conducting, which forward biases D11 while at the same timethe change in the state of the on-off flip-op causes D10 to be backbiased. However, at least one of the two diodes D10 or D11 is holdingT24 off so the lamp is held off. D10 will remain back biased so long asIlthe on-oif flip-flop is on. But D11 will only remain back biased aslong as the single shot multi-vibrator is in its unstable state. Thistime of being in the unstable state is determined by the time constantsof the single shot multi-vibrator, which, in the exemplary model, is setfor 60 to 180 milliseconds by control VRS. This controls the time of thedensitized interval (state 2). When both D11 and D10 become back biased,then T24, T23 and L1 are turned on. Thus, state 3, the sensitive state,is achieved.

Three transistors T-20, T-21, and T-22 control the on-otf flip-flop inrelation to the condition of the pilot lamp L-l. The transistor T-22 hasin series with its emitter the resistorR-S which senses current flow inthe lamp L-1 circuit. When the current sensed by the resistor R-86 isbelow a minimum required to illuminate the lamp L-l, it indicates thatthe pilot lamp L-l is burned out, and the electronic circuit protectorshould be turned olf. If normal current ows through the resistor R-86,`the transistor T-22 is cut off. When the transistor T-22 is cut off, thetransistor T-21 is cut off. When the transistor T-21 is cut off, thecoupling diode D-7 in the on-off flip-flop is back biased therebydriving the on-otf ip-flop to the off condition.

If there is inadequate current going through the resistor R-86 and ifthere is a signal into the other transistor T-20 through the resistorR-87 and a capacitor C-20, both the transistors T-20 and T-21 willconduct, driving the on-off flip-flop to the off condition through thediode D-7.

CONTROLLED SWITCH A transformer TRP-1 in the controlled switch circuitis used to provide voltage isolation between the load switch and itscontrol circuit. Each side of the output of the transformer TRP-1 iscoupled through a plurality of diodes D-20, D-21, D-22, and D-23, whicheffect full Wave rectification of the transformer output, the D.C.voltage being applied to the gate cathode input of a pair ofsilicon-controlled rectiers SCR-1 and SCR-2. Thus, when there is an A.C.signal of the proper frequency and magnitude at the input of thetransformer TRF-l, then there is a rectified D.C. signal at the twooutputs of the transformer which, in turn, forward bias the gate cathodejunction of the silicon-controlled rectifier SCR-1 and SCR-2.

In the absence of an input, the silicon-controlled rectiers SCR-1 andSCR-2 turn off at the rst current zero after the removal of power to thegate thereof.

A transient voltage suppressor TVS is inserted across thesilicon-controlled rectifier SCR-1 and SCR-2 to aid in clippingtransient voltages which might be developed thereacross.

The input to the silicon-controlled rectifier drive transformer TRP-1 isa SO-kc. square wave which is obtained from a drive circuit comprising afree running multi-vibrator. The output of the multi-vibrator is betweena resistor R- and a resistor R-126 that feed a transistor T-32. Thetransistor T-32, along with a transistor T-33, form a series And circuitthat produces a desired output sequence.

If the on-of flip-flop is in the on state, then this And circuit has aSO-kc. output.

The transistor T-32 has a resistor R-13'7 in its collector circuit thatis coupled through a capacitor C-3S, to the base of a transistor T-34driving a complementary emitter follower circuit consisting of thetransistors T35 and T-36.

A diode D-ZS is coupled to the base of the transistor T-34 to equalizeload on the capacitor C-35. The emitter of the transistor T-34 has arelatively constant Voltage bias provided by a diode D-26. The emitterfollower has a regulator circuit that consists of a capacitor C-36 inshunt with a transistor T-36. The purpose of the transistor T-36 and thecapacitor Ca36 is to provide a regulator circuit which provides forcontrol of the voltage source in response to the primary current ofTRP-1.

If the current to the drive transformer TRP-1 exceeds a certain level, acurrent limiting circuit reduces the current. A diode D-27, resistorR-13'5 and a capacitor C-39 couple the base of a transistor T-37 to theinput current of the transformer TRP-1 which is sensed by R-136. Theseelements sense current in the transformer TRP-1 and, in turn, controlthe transistor T-36 so as to limit the maximum current.

POWER SUPPLY The power supply is conventional in nature, except for theportion thereof forming a part of the threshold circuit. As seen inFIGS. 3 and 4, the threshold circuit includes a reference voltage ortrip mode switch S-1. The switch S-1 effects either the line voltagecompensated or constant current trip modes of operation.

In the constant current trip mode, a constant voltage is provided to thethreshold circuit that is generated by a power supply comprising aresistor R- and a zener diode ZD. The constant voltage output provides aconstant voltage reference to the threshold circuit, which correspondswith a constant current tripping point.

When it is desired to operate in the line voltage compensated mode, avoltage is provided which, for most practical purposes, may be assumedto have a linear relationship to line voltage. In other words, if linevoltage drops, the tripping point is lowered. This mode of operation isbased on the assumption that if an overload occurs, it bears apercentage relationship to normal rated current.

A transformer TRP-2 has a bridge rectifier in its secondary comprising aplurality of diodes D-34, D-35, D-36 and D-37. This along with C-40,R-152, VR-10,

From the `foregoing description, it should be apparent that theelectronic circuit protector incorporates an approach to the problem ofcircuit protection which materially differs from conventional circuitprotectors.

The electronic circuit protector provides close-in protection and fasttrip response, and yet it is capable of tolerating the large turn-ontransients associated with the input to transform circuits. The actualtrip current level of the-electronic circuit protector is variable overa. range of overloads.

The major system components of the electronic circuit protector are aswitch, and a control circuit which controls the sequence of operationsconstituting the turn-on cycle. This cycle is characterized byv varyingthe sensitivity of the circuit protector in what may be termed threechronologically related intervals, i.e., state one, state two, and statethree.

State one of the turn-on cycle is identified by complete tripinhibition, which means that the electronic circuit protector will nottrip, regardless of load size. It is to be noted, however, that thesilicon-controlled rectitiers SCR1 and SCR-2 are protected by a currentlimiting fuse F which opens the circuit on excessively large loads. Thisinactive portion of the turn-on cycle lasts for a preset time, forexample, two cycles.

State two involves an interval of 60 to 180 milliseconds, the exactvalue being an internal adjustment, in which the trip function operatesat a reduced sensitivity. The trip level for State two can be adjustedfrom, for example, 1000% to 5000% overload.

In State three, the protector operates at maximum sensitivity. Thisstate is the normal operating mode of the device, if the on-off resetswitch is on and trip has not occurred prior to this point, theprotector will remain in State three until either the power isdisconnected, or an overload occurs.

Whenever the instantaneous load current exceeds a predetermined level,the control circuit produces a trip signal to the controlled switch. Thecontrolled switch trips on the rst lload current zero subsequent to thetrip signal. The current necessary to trip the circuit protector isdependent only on the particular state involved when overload occurs,and is unrelated to load current prior to overload.

It is understood that the specific constructions of the improvedelectronic circuit protector herein disclosed and described arepresented for the purpose of explanation and illustration and are notintended to indicate limits of the invention, the scope of which isdefined by the following claims.

TABLE OF COMPONENT VALUES IN AN EXEM- PLARY CONSTRUCTED EMBODIMENTSenser Ohms Variable gain amplifier VR-l 2K.

VR-Z .l meg. VR-S 50K. R-S 2K.

R-S 20K. PC Clairex CL504L. A1 Philbrich P65 C-1 .001 pfd.

12 Generalized functionicircuit R-lt) 51K. R-lZ 51K. R-14 22K. A-2Philbrich P65. C-2 .001 pfd. D-l 1N461A.V D-2 1N461A.

Threshold circuit R-ZO 10K. R-22 150K. R-24 10K. R-26 10K R28 68K R-3015K R-32 68K R-34 9 1K R-36 10K. R-38 40K. T-l 2N336A T2 2N3363 T-32N1305 T-4 2N1305 C-4 .02 ,lfd C-6 .02 pfd S-l S.P.DT

Initial inhibiting circuit R-40 18K. R-42 47K. R-44 2.2K. R-46 22K. R-482.2K. T-S 2N1305. T-6 2Nl305. T-7 2N1305. C-8 2 pfd.

Input-on-o -reset S-2 S.P.D.T.

Switch control circuit R-50 22K R-SZ 22K R-53 2.2K R-S4 5.6K R-56 5.6KR-SS ohms-- 680 R-60 do 68 R-62 do 68 R-64 22K R-66 22K R-68 22K R-7022K R-72 5.6K R74 5.6K R-76 2.2K R-78 2.2K

D-3 1N461A D-4 1N461A D-S 1N461A D-6 1N461A D-7 1N461A D-8 1N461A D-91N461A C-9 .022 pfd. C-ltl 500 pfd. C-11 500 pfd. C-12 500 pfd. C-13 500pfd.

T11 2Nl305 13 TABLE- Continued T-12 2N1305 T-13 2N1305 T-14 2Nl305 T-152Nl305 T-16 2N1305 Generalized gain control function R-81 47K R-SZ 22KR-83 5.6K R-84 2.2K R-SS K R-86 ohms 39 R-87 5.6K R-88 1K R-89 ohms 680R-90 1.5K R-91 5.6K R-92 10K R-93 10K R94 10K R-95 ohms 15 R-96 d0 390R-97 do 330 R-98 10K R-100 1K R-101 1K R-102 10K R-103 5.6K

T-20 2N1305 T-21 2N1305 T-ZZ 2N1304 T-23 2N1305 T-24 2N1304 T-25 2N1305T-26 2N1305 C-20 .1,ufd. C-21 .02 lafd. C-zz .02 Mrd. C-23 .12 pfd. C-241.5 pfd. D-10 1N461A` D-ll 1N461A D-12 1N461A VR-S 100K Controlledswitch SCR1 2N687. SCR2 2N687. TVS GE6RS21SASD5.

D-20 1N461A D-21 1N461A D-22 1N461A D-23 1N461A D-24 1N461A D-25 2N1305D-26 1N2070 D-27 1N461A C-30 4 lafd. C-31 4 lafd. C-32 200 pfd. C-33 200pfd. C-34 100 pfd. C35 4 ,ufd. C-36 25 afd. C-37 4 lafd. C-39 4 /tfdR-120 `1K R-121 1K R-122 1K R-123 68K 14 TABLE-Continued R-124 68K R-125ohms 180 R-126 1K R-127 1K R-128 ohms 470 R-129 10K R-130 5.6K R-1315.6K R-132 1K R133 1K R-134 ohrns 220 R-135 do 220 R-136 do 3.3

T-30 2N1305 T-31 2Nl305 T-32 2N1304 T-33 2Nl304 T-34 2Nl305 T-35 2Nl305T-36 2Nl304 T-37 2N1305 T-38 2N1612 Power supply CB Mechanical productsMP 1500 .5 amp. ZD 1NS25.

D-31 1Nl692.

D-32 lN692.

D-34 1N461A.

D35 1N461A.

D-36 1N461A.

D-37 1N461A.

C-40 25 lttfd. C-41 6000 ttfd. C-42 6000 afd. C-43 100 Mrd. C-44 2 afd.C-45 100 pid. C-46 2 lafd. C-4'7 .02 fa R- ohms 680 R-151 do 100 R-152d0 20 VR-10 5K TRF-Z RT201 TRF-3 P6465 F Chase-Shawmut 15 amp. 250 v.

What is claimed is:

1. In an electrical circuit comprising a source of electrical energy anda load, an electronic circuit protector for interrupting the electricalcircuit upon the occurrence of a predetermined condition therein, saidelectronic circuit protector comprising a signal responsive switchelectrically connected to the source and load, means for sensingelectrical conditions in the circuit having an output signal, a switchcontrol circuit normally responsive to the output from said means forcontrolling the condition of said switch, an inhibiting circuit forinhibiting the output of said sensing means for a predetermined initialperiod subsequent to turn-on of said electronic circuit protect-or, anda desensitizing circuit for desensitizing said switch control circuitfor a predetermined period of time subsequent to the inhibited period.

2. An electronic circuit protector for interrupting an electricalcircuit upon the occurrence of a predetermined condition therein, saidelectronic circuit protector comprising a signal responsive switchelectrically connected in said circuit, and aI control circuit lhaving atime dependent characteristic for controlling the condition of saidswitch, said control circuit comprising means 4for sensing an electricalcondition in the circuit having an output signal, a variable gainamplifier for controlling the variations of the'output of said sensingmeans, a generalized function circuit for modifying the output from thevaria-ble gain amplifier in accordance with a desired function, athreshold circuit for comparing the output of said generalized outputfunction circuit with a reference output, an on-offreset control switchfor providing on and off control of the controlled switch and reset ofthe control circuit, a switch control circuit for determining the stateof the signal responsive switch in response t-o the state of theon-offreset switch and the threshold circuit, and a generalized gaincontrol function for controlling variations of the variable gainamplifier in -relation to time.

3. An electronic circuit protector for interrupting an electricalcircuit upon the occurrence of a predetermined condition therein, saidelectronic circuit protector comprising a signal responsive switchelectrically connected in said circuit, and a control circuit having atime dependent characteristic for controlling the condition of saidswitch, said control circuit comprising 'means for sensing an electricalcondition in the circuit having an output signal, a variable gainamplifier for controlling the variations `of the loutput of said sensingmeans, a generalized function circuit for `modifying the output from thevariable gain amplifier in accordance with a desired function, athreshold circuit f-or comparing the output of said generalized outputfunction circuit with a reference output, an on-off-reset control switchfor providing on and off control of the controlled switch and reset -ofthe control circuit, a switch control circuit for determining the state-of the signal responsive switch in response to the state of theonoff-reset switch and the threshold circuit, and a generalized gaincontrol function for controlling variations of the Variable gainamplifier in relation to load.

4. An electronic circuit protector for interrupting an electricalcircuit upon the occurrence of a predetermined condition therein, saidelectronic circuit protector cornprising a signal responsive switchelectrically connected in said circuit, and a control circuit having atime fdependent characteristic yfor controlling the condition of saidswitch, said control circuit comprising means for sensing an electricalcondition in the circuit having an output signal, a variable gainamplifier for controlling thevariations `of the output of said sensingmeans, a generalized function circuit for modifying the output from thevariable gain amplifier in accordance with a desired function, athreshold circuit for comparing the output of said generalized outputfunction circuit with a reference output, an on-off-reset contr-olswitch for providing on and off control of the controlled switch andreset of the control circuit, a switch control circuit for determiningthe state of the signal responsive switch in response to the state ofthe on-offreset switch and the threshold circuit, and a generalized gaincontrol function for controlling variations of the variable gainamplifier in relation to time and l-oad.

5. An electronic ci-rcuit protector for interrupting an electricalcircuit upon the occurrence of a predetermined condition therein, saidelectronic circuit protector comprising a signal responsive switchelectrically connected in said circuit, and a control circuit having atime dependent characteristic for controlling the condition of sai-dswitch, said control circuit comprising means for sensing an electricalcondition in said electrical circuit having an output signal, athreshold circuit for triggering upon the occurrence of saidpredetermined condition, a generalized function circuit for modifyingsaid sensor means in accordance with a predetermined function, and avariable gain amplifier connected in said control circuit to controlgain between said sensor and said threshold circuit for determining thetrip characteristic.

References Cited by the Examiner UNITED STATES PATENTS 2,551,022 5/ 1951Larstrup 317-49 3,079,533 2/ 1963 Kothei-mer 317-31 X 3,160,788 12/1964Antoszweski et al B17-36 MILTON O. HIRSHFIELD, Primary Examiner.

SAMUEL BERNSTEIN, Examiner.

I. D. TRAMMELL, Assistant Examiner.

2. AN ELECTRONIC CIRCUIT PROTECTOR FOR INTERRUPTING AN ELECTRICALCIRCUIT UPON THE OCCURRENCE OF A PREDETERMINED CONDITION THEREIN, SAIDELECTRONIC CIRCUIT PROTECTOR COMPRISING A SIGNAL RESPONSIVE SWITCHELECTRICALLY CONNECTED IN SAID CIRCUIT, AND A CONTROL CIRCUIT HAVING ATIME DEPENDENT CHARACTERISTIC FOR CONTROLLING THE CONDITION OF SAIDSWITCH, SAID CONTROL CIRCUIT COMPRISING MEANS FOR SENSING AN ELECTRICALCONDITION IN THE CIRCUIT HAVING AN OUTPUT SIGNAL, A VARIABLE GAINAMPLIFIER FOR CONTROLLING THE VARIATIONS OF THE OUTPUT OF SAID SENSINGMEANS, A GENERALIZED FUNCTION CIRCUIT FOR MODIFYING THE OUTPUT FROM THEVARIABLE GAIN AMPLIFIER IN ACCORDANCE WITH A DESIRED FUNCTION, ATHRESHOLD CIRCUIT FOR COMPARING THE OUTPUT OF SAID GENERALIZED OUTPUTFUNCTION CIRCUIT WITH A REFERENCE OUTPUT, AN ON-OFFRESET CONTROL SWITCHFOR PROVIDING ON AND OFF CONTROL OF THE CONTROLLED SWITCH AND RESET OFTHE CONTROL CIRCUIT, A SWITCH CONTROL CIRCUIT FOR DETERMINING THE STATEOF THE SIGNAL RESPONSIVE SWITCH IN RESPONSE TO THE STATE OF THEON-OFFRESET SWITCH AND THE THRESHOLD CIRCUIT, AND A GENERALIZED GAINCONTROL FUNCTION FOR CONTROLLING VARIATIONS OF THE VARIABLE GAINAMPLIFIER IN RELATION TO TIME.